1. Field of the Invention
The present invention relates to a double casing type pump provided with: a rotation shaft; at least two impellers provided on the rotation shaft; an inner casing shrouding the impellers; an outer casing shrouding the inner cover and having a suction opening and a discharging opening regarding the working fluid of the pump. The present invention also relates to a performance adjusting method regarding the double casing type pump.
2. Background of the Invention
In recent years, the need for double casing type pumps and the head adjusting method thereof is increasing, the pump being provided with: a rotation shaft; at least two impellers provided on the rotation shaft; an inner casing shrouding the impellers; an outer casing shrouding the inner cover and having a suction opening and a discharging opening regarding the working fluid of the pump.
In a case where the rotation speed of the pump is constant, providing an opening-adjustable control valve (a control valve which opening is adjustable) on the fluid discharge line of the pump is well known as a method for adjusting the performance of the pump. However, according to this method, providing the control valve on the fluid discharge line causes a pressure drop and the deterioration of the pump efficiency. In this regard, the need for a feasible method for adjusting the performance of the pump other than the above-described method (providing the opening-adjustable control valve) is increasing.
Conventionally, in adjusting or modifying the pump performance, the adjustment of the outer diameter or the geometry of the impellers, the correction of the volute fluid-passage of impellers, or the modification of the shape of the volute fluid-passage is performed.
However, in performing each of the adjustment of the outer diameter or the geometry of the impeller, the correction of the volute fluid-passage and the modification of the volute fluid-passage, it becomes necessary to disassemble the pump, replace the impellers or the inner casing, or re-machine the impellers or the inner casing. Thus, the performance adjustment after the completion of the pump accompanies large-scale disassembling and re-assembling work; the performance adjustment needs a considerably large amount of hours and costs. In a case where the performance adjustment is performed particularly at the site after commissioning, it is often difficult to re-machine the impellers or the inner casing disassembled at the site; thus, it often becomes necessary to replace the components by substitute ones at the site. Hence, the performance adjustment often needs a huge amount of hours and costs particularly at the site.
Further, in performing the performance adjustment of the pump, when the impeller is replaced or re-machined and the whole balance of the rotor is reconditioned, then re-examining the vibrations of the reconditioned rotor is required; thus, a huge amount of hours and cost becomes necessary for the re-examination as to the rotor vibrations.
Further, when the rotor reconditioning is performed so as to adjust the pump performance, a possibility arises where each pump has different rotor geometry. Thus, the interchangeability regarding the components is lost between a component of a pump and the corresponding component of another pump that is manufactured as a same type pump in a series-production.
In the background as described above, as a technology that eases the pump performance adjustment, the patent reference 1 discloses a pump having an impeller provided with a plurality of main impeller vanes; whereby, an additional vane is provided between the back side (i.e. belly surface side) of a main impeller vane and the front side (i.e. back surface side) of the adjacent main impeller vane; the additional blade is extended from a point on the back side surface of the impeller vane toward the front side surface of the adjacent impeller vane. According to this disclosed technology, the pump performance curve of the pump can be easily adjusted thanks to the impeller with the additional vanes.
Further, the patent reference 2 discloses a technology regarding a power steering apparatus; thereby, the pressurized fluid discharged from the pump is sent to a power steering apparatus via a throttle passage; by returning a surplus fluid flow to the suction side of the pump by use of a flow rate control valve that controls the degree of the opening regarding a by-pass passage through which the surplus fluid flow returns to the suction side.